Radio-frequency power measuring system



L. E. NORTON RADIO-FREQUENCY POWER MEASURING SYSTEM July 8, 1952 2 Sl-IEETSSHEET 1 Filed Aprii 29, 1947 Lowell 121 333 ATTORNEY IA/D/CWTOR 2 SI'IEETS-SHEET 2 July 8, 1952 1.. E. NORTON RADIO-FREQUENCY POWER MEASURING SYSTEM Filed April 29, 1947 LO/ID Wow/(70K FILM ELEMENT AAIPUF/ER GENE/6470K vention provide satisfactory wave measurements sensitivity and introduce negligible wave discontinuities into the transmission line, but due to the fact that they are dependent upon a variable division ratioof circulating currents in the transmission line conductors to currents in the thin film, they are somewhat 'frequency dependent and a correction factor must be employed if the apparatus is utilized over extremely wid transmission frequency ranges.

A preferred embodiment of the invention, adaptable to either coaxial line or waveguide transmission systems, utilizes a semi-conductive film supported between extremely .thin mica sheets and disposed transversely of the axis of wave propagation in the transmission system. Due to the extremely small axial dimensions of the supporting and energy absorbing elements disposed transversely of the transmission line, wave discontinuities may be substantially disregarded and thus wave reflections are not objectionable. Since the system employing a transverse energy absorbing element is not dependent upon a division of circulating currents in the transmission line conductors and in the thin film, the energy response is substantialy independent of the operating frequency up to frequencies at least as high as 10,000 megacycles. lhe low frequency limit is determined only by the capabilities of the radio frequency energy transmission system and by the fact that the semi-conductive film is of necessity capacitively coupled to the conductors of the transmission line. In the latter embodiment of the invention, connections are made to opposite sides of the semi-conductive film for connection to the indicating apparatus. 'It should be understood that the shape of the semi-conductive film and its supporting elements in each of the embodiments of the invention disclosed herein may be varied as de sired to suit the individual transmission line conformation and transmission characteristics. Furthermore, the thickness of the semi-conductive film and its supporting collodion or mica elements are determined by the maximum frequency for which power measurements are to be made and by the maximum permissible thermal mass to provide the desired short thermal time constant.

Among the objects of the invention are to provide an improved method of and means for measuring radio frequency energy in a wave transmission system. Another object is to provide an improved method of and means for providing radio frequency power measurements which are substantialy independent of the oper- I ating frequency. A further object is to provide an improved method of and means for measuring radio frequency power in a radio frequency transmission system with minimum wave discontinuity and wave reflections due to the measurement apparatus. An additional object is to provide an improved wide frequency band radio "frequency power measurement system of high sensitivity and power efficiency. 'A still further object of the invention is to provide an improved wide band radio frequency power measurement system which introduces negligible wave reflections and absorbs an extremely small portion of the transmitted energy from the transmission line, wherein the measurement apparatus has an extremely short thermal time constant and is capable of power measurements over a widepower range. A still further object of the invention is to provide an improved wide frequency band radio frequency power measurement system operable with either coaxial line or waveguide transmission systems. Another object of the invention is to provide an improved wide frequency band radio frequency power measurement system utilizing a wave sensitive element cornprising a film of semi-conductive material such as tellurium-zinc alloy which is thin with respect to the skin depth of radio frequency currents at the operating frequencies.

The invention will be described in greater detail by reference to the accompanying drawings of which Figure 1 is a top plan view of a first embodiment of the invention adapted to the measurement of radio frequency power in a coaxial transmission line system; Figure 2 is a cross-sectional elevational view taken along the section line II-II of Fig. 1; Figure 3 is a top plan view of another embodiment of the invention adapted to the measurement of radio frequency power in a waveguide transmission system; Figure 4 is a cross-sectional view, taken along the section line IV-IV of Fig. 3; Figure 5 is a cross-sectional elevational View, taken along the section line V-V of Fig. 6, of a portion of the devices shown in Fig. 2 or 4 and including a metal shield and supporting ring for the wave detecting element; Figure 6 is a bottom view of a modification of the device shown in Fig. 5 employing radial shielding elements within the indicator transmission line; Figure '7 is a crosssectional View of a preferred embodiment of the invention adapted to radio frequency power measurements in a coaxial line transmission system; Figure 8 is an enlarged, exploded fragmentary view of a portion of the device shown in Fig. 7; Figure 9 is a bottom view of the device shown in Fig. 8, Figure 8 being a cross-sectional view taken along the section line VIIIVIII of Fig. 9; Figure 10 is a cross-sectional view of still another preferred embodiment of the invention employing a transverse measurement element in a waveguide transmission system; Figure 11 is a cross-sectional elevational view taken along the section line XI-XI of the device of Fig. 10, Figure 10 being a cross-sectional view taken along the section line X-X of Fig. 11; Figure 12 is a block circuit diagram of a measurement system employing the system, and Figure 13 is a schematic circuit diagram of a bridge measurement indicating system employing the invention. Similar reference characters are applied to similar elements throughout the drawings.

Referring to Figures 1 and 2 of the drawings, a first embodiment of the invention adapted to measurements of radio frequency power in a coaxial trasmission line having inner and outer conductors 3, 5 respectively, comprises a semiconductive film device 3 opening into an aperture 9 in the outer conductor 5 of the transmission line. A semi-conductive film ll comprising, for example, a deposit of tellurium-zinc, having a thickness less than the skin depth of the radio frequency currents at the operating frequencies, is supported by a similarly thin film l3 of collodion which in turn is supported at the end of a cylindrical conductive element 95. A center conductor 1'! issupported on the axis of the cylindrical conductor l5 by an insulating spacer is to form an indicator circuit coaxial line. The center conductor ii of the indicator coaxial line 1 passes through an aperture 2! at the center of the collodion film l3, and is connected to the semiconductive film H by a drop of Aquadag or other soluble conductive material.

It should be understood that the semi-conductive film I 3 may be formed "as an annuiar rmg or as a longitudinal strip as indicated in' Figure 1, or that other cross-sectional shapes may be employed. The combined'thickness of the semiconductive film II and collodion supporting film I3 should not exceed a few thousand angstrom units in order that the element may have small thermal mass and short thermal time constant. The collodion film I3 should be as non-dissipative as possible at themaximum microwave frequency employed. Such thin collodion films may be secured by droppingextremly dilute collodion on a water surface, and lifting the film from the water surface by contact with the "end of the cylindrical indicator line conductor I5. The semiconductive film II of tellurium-zinc or the like then may be evaporated to the desiredthickn'ess and density directly'upon the collodion film. By control of the conductivity and thickness of the semi-conductive film, it is possible to choose the fraction of the total transmitted power in'the transmission system which will be dissipated in the 'film.

The two connections to the film, the inner and outer coaxial conductors'I I, I respectively of the indicator line I thence may be 'conne'cted to the measurement apparatus for observing the re'sistancechange or the absorbed microwave power in response to microwave energy transmitted through the coaxial transmission line I.

The semi-conductive film may be connected as warm of'a four element A.-C. .bridge'whereb'y itsvariation in resistance in response to'temperature'changes due to absorbed'microwave energy may be readily determined, and the transmitted microwave power calculated therefrom. Alternativelythe semi-conductive film I I may be coupled through the indicator transmission line I5, IT to a wave detector and a voltage determining circuit which measures the microwave voltage induced between the terminals of the film in response to absorbed microwave-energy. Sinc'ethe resistance of the tellurium zincalloy film is quite non-linear withapplied potential, it is desirable toconnectit to a narrow band, high gain amplifier in order to avoid spuriousindications due to higher orderharmonics which would disturb the linearity of the indicating system.

Figures 3 and 4 indicate a typical method of utilizing the semi-conductive film device of Figs. '1 and -2Jfor'measurement of transmitted energy through a waveguide 25; A circular opening TI is-cut into the'upper wide face of the waveguide 2'55and-the indicator coaxial line I5, I! supporting the semi-conductive film II is securedto the upper waveguide face by means of a circular fian'ge29. The semi-conductive film II may be circular in shape or, if desired, may be any other desired shape 'to minimize undesired localresonance effects.

Figures 5 and '6 show modifications of the indicator coaxial line and semi-conductive film structure wherein the collodion film I3is backed by'a metal shield 3| in contact with the outer cylindri'cai conductor I5 of the indicator line but having anaperture at its center spacing it from'the center indicator conductor I'I. Figure 6 illustrates a modification of the metal shield 3| comprising aplurality-of inwardly radially'extending metallic'arms "33 connected to the outer indicator line conductor I 5 but separated from the 'central'indicator'line conductor I'I. The shielding effect of the radial arms v33 is quite satisfactory for all practicable operating frequencies of the transmission line and may be uti1ized' to e1iminaterm; desired local resonance effects'in the metal shield. To determine the fraction *of the total transmitted power which is dissipated in the semiconductive film, "the-normal termination of the coaxial 'transmissi'onf'line I is replaced by a power measuring device of any type known in the art which absorbs all of the 'transmitte dpower applied thereto. The indications provided by the semi-conductive film thus maybe readily eali brated. Another method-of determining thefraction of power absorbed by the semi-conductive film is by substituting known resistancesiinithe measurement bridge to provide balance thereof for different :in'cremental values-of the-semi conductive film resistance for known values of power supplied to the transmission line terminaetion. From the known temperature-resistance coe'fiicient of the "semi c'onductive material, the corresponding temperature :rise therein may be computed. "From the dimensions and geometny of the semi-conductive film element, the power necessary to produce such temperature variations may be calculated. 4

This calculation; ormeasurement, orth'e frac tionof the total power in the system which is dissipated in the film need only be made during initial adjustmentandicalibration .of the system.

For a terminationwhich 'Provid'esunity standing-wave-ratio in the transmission system the device then provides an output voltage which is proportional to the square Of the magnetic field density'at the.fi1m,'(ir proportional to'the power in the system if the'indicator system is linear.

For any other terminationgiving-any"other standing wave ratio it is necessary to measure with a wave probe the standing wave ratio and the relative field value at "the film as measured with the probe rm/2 from the film. With this information it is possible to calculate the power from the indicated reading.

Since, as is wellfknown, the fields in a waveguide system have a frequency dependence it is advantageous to substitute the coaxial transmission system, which'is'inade to "propagate only the principal mode. The dissipative film- 'i'smo'iinte'd on a cylindrical unit but itisconv'enientto make the film in strip or ribbon form instead of circular cross-section;

The systems of Figures 1 to 6 provide power measurements which have a frequency depend-1 ence which exists for the following reason:

The material of conductivity a1 and thickness d is embedded in the outer wall of a coaxial line of higher conductivity 0'2 and a thickness greater than skindepth.

If the current'density'at the surface (ate-=0) is in; then The totalrcurrent inthe semi-conductorls I The average power absorbed per unit length of the outer conductor, thick on a skin depth scale, and of inner diameter h is where m is the resistance per unit length of oute conductor, so that v. .v

7 Since the two conductors are approximately in parallel for longitudinal current flow 2 I 2 P =(1 T T 1 tn+rz l '2 If as is always the case, T1/12 1 N 2 22) P.=I.

and from (8) Pl=It T2 G1CZ (14) From (12) T i nfra 2111) 2 so that lfl i fifum Pl 41r b a 4717 0; (15) which indicates that the power dissipated in the thin film of conductivity a1 is proportional to the frequency, f. This conclusion is based upon the assumption that a thin semi-conductive film has only the properties of a scaled down thin section of conductor. There is some evidence, although rather incomplete, that evaporated thin films'of certain metals exhibit either an equivalent series inductance or shunt capacitance effect which, over a predetermined frequency range substantially eliminates the power-frequency dependence. However, in general the foregoing conclusions regarding frequency dependence tend to limit the useful frequency response or" the system.

For this'reason, the broadband operation the embodiments of the invention shown in Figures 7 to 11 are preferred. The thin semi-conductive film in the form of a circular disc is placed normal to the axes of the conductors of the coaxial line or to the direction of wave propagation of a waveguide transmission system. In the coaxial line, preferred embodiment of the invention, the disc is mounted on a thin supporting membrane (mica is satisfactory if thin) supported on the end of a glass cylinder. This assembly is inserted in a groove in the thick walled outer conductor. The glass is not exposed to the radio frequency fields. Contact leads are brought out on opposite sides of a diameter by platinum and silver surfaces painted and baked on the sides of the glass and then to metal terminals sealed in the glass.

A preferred embodiment of the invention which is substantially independent of operating frequency and which is adapted for measurement of transmitted power in a coaxial transmission line, utilizes a semi-conductive film supported between extremely thin mica wafers and disposed transversely of the axis of Wave propagation in the coaxial transmission line. Referring to Figures '7, 8 and 9, the measurement apparatus includes a pair of input and output coaxial line connectors 4!, 43 adapted for serial connection in a coaxial power transmission line, not shown. The outer shells 45, 47 respectively of the connectors 4|, 43 are connected to cylindrical conductive elements 49, 5| respectively, which are aligned by annular plates 53, 55 secured together by screws 51. The center conductor of the coaxial line in the measurement device includes a pair of telescoped conductive rods 59, 6| which screws 99.

9. are terminated in the connectors 4|, 43, respectively.

The semi-conductive film'element II is supported between thin parallel disposed mica sheets 63-, 65, which are apertured for passage of a reduced diameter portion 61 of the inner coaxial line conductive rod- 59. The-telescopic portions 59, 6| of the-innercoaxial line conductor-thus may be telescoped tocontact the respective mica sheets 63, B5. The semi-conductive film H includes a central aperture 69 sufficiently large to provide clearance for the reduced diameter portion 6] of the centerc rductorof the coaxial line. Themica sheets. 63, {SE-enclosing theisemiconductivefilm H are cementedtoone end of: a cylindrical insulating-member it having metallic contacts 13, molded therein and connected through insulated conductors ll, 19 to opposite sides of the semi-conductive film. The connections to the film may be made by silver solder and silver plating to the edges of the film.

The insulated supporting structure H carrying the semi-conductive film is inserted into a keyhole slot 8! milled into the outer coaxial con.-

ductor l9 coaxially with the aperture therethrough. Connection leads. from the terminals '13, 15 are brought out through apertures adjacent thereto in the outer conductor 490i. the coaxial line device. An additional enclosure comprising an angular member 83 and coaxialshell 85-may beutilized if desired.

Great caremust be. exercised in the assembly of. thedevice, since the semi-conductive film H supportedbetween the mica sheets 63, 65 must not be objectionably distorted in assembly to the two sections 55, 91 of a waveg'uidetransmission system. The flanges aresecured together by Connections HH, Hi3 are provided toopposite sides ofthesemi-conductive filml 1. Such .connections may be provided by platinum ofsilver. surfaces pointed ontheedges of-the semi-conductive filmand silver soldered .to suit.- able conductors. The clamping screws Siishould beinsulated from the semi-conductive filmby clearance 1 apertures therein.

Figure .12 indicates schematically a. typicallcircuit. for employing the semi-conductive filmelement in a microwave transmission. system. A microwave generator ll] connected, through a waveguide or coaxial transmission, line I I3. and the. semi-conductive film element H5. supplies radio frequency energytoa.loaddevice.l I]. The connections fromthe semi-conductivefilm element. H5 are connected .to ahigh gain, narrow 'bandamplifier H9, the output of which. isconnected to an. indicator l2! which may be calirbratedin terms of the energy appliedtothe load I H. such a system depends upon the energy induced in the semi-conductive film element in response to energy transmitted through the load line H3. If the load element Ill isnot properly matched to the-transmissionline ll 3,.standing waves will result thereon and it is essential that the standing wave ratio be known inorder.

that the load power may be computed. The standing wave ratio may be determined by a longitudinally movable waveprobe I23, of any known type, the position of which may be adjusted to provide wave magnitude indications at a point ni/Z removed from the semi-conductive film. With this information it is possible to calculate the transmitted power from the indicated reading if the devicehasbeen calibrated asv described heretofore. For a load terminationv whichprcvides unity standing wave ratio in the transmission system, the movable probe E23 may be omitted and the indicator maybe calibrated to provide an output which is-proportional tothesquare of the magnetic field density at. thefilm or proportional to the power. in-the system if'the amplifier and indicator devices provide linear response.

Figure 13 shows an alternating current bridge circuit for measuring the variation in resistance of the semi-conductive film element in response to radio frequency energy dissispated therein. The calibration of such a system has been described heretofore. Once the system has been calibrated, a D.-C. calibrationsystem. may be utilized to avoid errors due to variations in over all gain and to avoid errors due to drift. A bridge network comprising a variable resistor [23 and two fixed resistors [25' and I2! includes the semiconductive film element l l as its fourth arm. A source of alternating potential connected through a transformer I29 is coupled to opposite points 13!, I33 of the bridge. A high gain, narrow band amplifier I35 is connected to the other remaining balancedpoints I31, 139 of the bridge. The output of the high gain narrow band amplifierj35 is connected to an indicator I37 which indicates bridge balance.

The D.-C. calibration. circuit may include a battery I39 connected through a variable resistance [41 ,..a meter I43 and a switch l-45 to apply a predetermined D.-C; voltage to the points I31, I39 of the bridge. As explained heretofore, when a bridge balance has'beenobtained b'y' ad justment of the bridge variable resistor I23, the value of the resistance of the semi-conductive element may be determined by substituting a known resistor in its place in the bridge network which will maintain the bridge in a balanced condition. Another method which will provide satisfactory indications of transmitted power is to calibrate the variable bridge resistor 23 in terms of transmittedmicrowave power.

For the case. of the thin: absorbingfilm placed ina plane normal to thediretion of propagation the following situation exists. Using ordinary transmission line terminology, the input impedance at the left hand face. of the-thin filmis J Assuming the line to-be matched, ZR is real and is merely the characteristic impedance of the line on the load side of the film, Zo-is' the complex characteristic impedance or ther'sli'ort thinfilm, and Zr for matched conditionsi's only the characteristic impedance. ofthe. line on. the generator side of the thin film. Zn can be made equal to ZR, so that Z sinh yl-l-ZE; cosh 'yl' For the case where the line is matched, ZR is of the form where and so have the dimensions of permeability and dielectric constant, respectively, and m is a numerical constant depending on the transmission line geometry. For the case where the thin film has the same inner and outer diameter ratio as the line, and is in eifect an extension of the line but with different values of a and e, the numerical constant 172 appears as a common coefficient for all terms, and divides out. Whereas the system was first considered on a "characteristic imepdance, Z0 basis, the system then is considered on an /6 p. intrmsic impedance, basis As is well known, the dielectric properties of the conducting film may be represented by the complex expression so that the intrinsic impedance is where 01 is the conductivity. Also yr the propagation constant, is

' V 71=i [#i( 1% 2 For the film material most frequently used, a tellurium-zinc alloy, with a conductivity only about 0.86x10- that of copper and operating at frequencies as high as 10 cycles/sec.

Using these values in (17) U/45" sinh (Tl /45)+n cosh (Tl 45 2 Ugg cosh (Tl/45)+n sinh (Tl/45) Since n is real and positive it is obvious that (23) can be true, exactly, if

Tl/45 is zero For |TLl=0, (23) reduces to The maximum film thickness, 1, for this approximation follows by use of (28) in (20).

t c-er The conductivity, c1, of the tellurium-zinc alloy is 500 mho/m. For a maximum frequency of f=10 cycles/sec. and assuming that {L1 and e1 are not greatly different from ,uo and co in free space Expressed in angstrom units z=s.1s 10 A.

which is the maximum thickness of the telluriumzinc film to fulfill the approximation of (26) and (27).

For the film equal to or less than about l=3.2 10 A. the discontinuity and resulting refiection at the film is slight, and from continuity the longitudinal current must equal the longitudinal current in the line.

Thus the invention disclosed and claimed here in comprises an improved wide frequency band power measuring system for radio frequency signals in a coaxial or waveguide transmission system wherein the wave sensitive element comprises an extremely thin film of semi-conductive material providing negligible wave discontinuities in the transmission system and having sufilcient sensitivity to provide power indications with negligible power dissipation. The system disclosed provides accurate power indications for all frequencies from the lowest value which may be transmitted through the transmission system to frequencies in excess of 10,000 megacycles.

I claim as my invention:

1. A wide frequency band device for measuring radio frequency energy comprising a radio frequency transmission line coupling a source to a load, a semi-conductive sheet element having a thickness less than the radio frequency current skin depth in said element at the highest operatmg frequency of said energy, and means for couplmg said element into said line for subjecting said element to said energy to vary the impedance of said element, whereby the impedance of said element varies as a function of the energy absorbed thereby.

2. A device according to claim 1 wherein said element comprises a substantially microwave transparent support having a semi-conductive coating on one side thereof 3. A device according to claim 2 including means for shielding said element from external fields other than said radio frequency energy to be measured.

4. A device according to claim 2 wherein the thickness of said support is less than 500 A. and the thickness of said coating is less than 2000 A.

5. A device according to claim 2 wherein said coating comprises tellurium-zinc.

6. A wide frequency band device for measuring radio frequency energy in a transmission line comprising a semi-conductive sheet element having a thickness less than the radio frequency current skin depth in said element at the highest operating frequency of said energy, means for coupling said element into said line for subjecting said element to said energy, and connection means for an indicator for indicating the energy absorbed by said element.

7..A wide frequency band device for measuring radio frequency energy transmitted through wave transmission line from a source to a load comprising a semi-conductive film element, thin with respect to the radio frequency current skin depth in said element at the highest operating frequency of said energy, means for coupling said element into said line for subjecting said element to a portion of said transmitted energy to vary the impedance of said element, and connection means for an indicator for indicating said energy in response to said impedance variation of said element.

8. A wide frequency band device for measuring radio frequency energy transmitted through a waveguide from a source to a load comprising a semi-conductive sheet element having a thickness less than the radio frequency current skin depth in said element at the highest operating frequency of said energy, means for subjecting said element to said energy in said waveguide to vary the impedance of said element, and connection means for coupling to an indicator for indicating said energy in response to said impedance variation.

9. A device according to claim 8 wherein said element effectively comprises a portion of the walls of said waveguide.

10. A device according to claim 8 wherein said element is supported within said waveguide transversely to the axis of energy wave propagation therein,

11. A wide frequency band device for measuring radio frequency energy transmitted through a coaxial line from a source to a load comprising a semi-conductive sheet element having a thickness less than the radio frequency current skin depth in said element at the highest operating frequency of said energy, means for subjecting said element to said energy in said coaxial line to vary the impedance of said element, and connection means for coupling to indicaating means for indicating said energy in response to said impedance variation.

element for shielding said element from fields other than said radio frequency energy in saidv first line.

16. A device according to claim 11 wherein said element is supported within said coaxial line transversely to the axis of wave energy propagation therein.

1'7. A wide frequency band device for measuring radio frequency energy comprising a substantially energy transparent film support and a semi-conductive film element deposited on said support, the combined thickness of said support and said element being less than the radio frequency current skin depth in said element and support at the highest operating frequency of said energy, means for subjecting said support and element to said energy to vary the electrical characteristics of said element, and connection means for coupling said element for indicating said energy in response to said variation of said characteristics.

18. Apparatus according to claim 17 including an enclosed transmission line coupling a source of said energy to a load, said element being coupled to said energy in said line, an electrical bridge network including said element, amplifying means connected between said bridge network and said indicating means, and means for calibrating said indicating means to provide indications of the energy coupled to said load.

19. A wide frequency band device for measuring radio frequency energy in a transmission line comprising a support and a semi-conductive sheet element supported by said support interposed in said line and subjected to a portion of said energy, the combined thickness of said support and element being less than the radio frequency current skin depth in said element at the highest operating frequency of said energy, whereby variation of the impedance of said element in response to absorption therein of said energy is indicative of the amount of said energy.

LOWELL E. NORTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,106,768 Southworth Feb. 1. 1938 2,151,118 King et a1. Mar. 21, 1939 2,197,123 King Apr. 16, 1940 2,366,614 Hansell Jan. 2, 1945 2,399,481 George Apr. 30, 1946 2,429,200 Bradley et a1. Oct. 21, 1947 2,432,199 Kamm Dec. 9, 1947 2,441,165 Ovrebo May 11, 1948 2,464,277 Webber Mar. 15, 1949 2,564,706 Mochel Aug. 21,1951

OTHER REFERENCES Article, Microwave Measurements and Test Equipments, by F. J. Gaffney, published in Proceedings of the I. R. E., vol. 34, no. 10, October 1946. Copy in 178-44-1D. 

